Populus deltoides is a common riparian tree species in southeastern North America. Populus forms root associations with arbuscular, ectomycorrhizal, and endophytic fungi. To elucidate the structure of rhizospheric fungal assemblages on Populus deltoides we carried out a series of field campaigns in natural P. deltoides populations in NC and TN. Field studies were coupled with trap-plant experiments using cuttings of Populus. Fungal root communities were characterized through 454 amplicon pyrosequencing. Efforts were also made to culture fungi from Populus roots and characterize the arbuscular mycorrhizal community through spore studies. Our results indicate that in addition to hosting mycorrhizal taxa, P. deltoides supports a high diversity of fungal root endophytes. In fact, endophytic fungi accounted for the majority of sequences in field and trap-plant samples, and pure culture isolates from roots. There was considerable overlap between datasets. Well represented were ascomycete taxa belonging to the Hypocreales, Chaetothyriales, Pleosporales and Heliotiales, and basidiomycete taxa belonging to the Agaricales, Polyporales, and Atractiellales. Spore and molecular data indicate that Glomerales and Paraglomales are the main arbuscular mycorrhizal associates of P. deltoides. Although fruitbodies of ectomycorrhizal taxa were uncommon in the field, a new truffle species was collected under P. deltoides in TN and NC field sites and has been described (Tuber mexiusanum). Additionally, fruitbodies of Laccaria, Cortinarius, and Geopora spp. were produced in our trap-plant experiments. Other ectomycorrhizal taxa recovered in our molecular surveys included Inocybe, Hebeloma, Thelephora, and Russula spp. Pure cultures of Laccaria and Thelephora spp. have been established
